A wire electric discharge machining apparatus is an apparatus in which a wire electrode is placed opposite to a workpiece, pulsed discharge is generated in a machining gap between the wire electrode and the workpiece, to machine the workpiece into a desired shape by using a thermal energy. Normally, in wire electric discharge machining, crude machining is performed first, and then finish machining is performed for a plurality of times to improve shape precision.
In a corner portion of a machining shape, machining accuracy tends to decrease as compared with a linear portion, which is formed by relatively moving a wire electrode in a linear direction. The corner portion is formed by changing a direction of relative move of the wire electrode, and is machined by relatively moving the wire electrode along a circular arc trajectory. For example, in a case of in-corner machining in which a wire electrode is moved inside of a corner portion, because a machining amount increases between a previous machining surface and a current machining surface, as compared with machining of a linear portion, left over easily occurs in the corner portion. Further, in a case of out-corner machining in which a wire electrode is moved outside a corner portion, because a machining amount decreases as compared with machining of a linear portion, excessive machining in the corner portion easily occurs.
To improve reduction of machining accuracy, control is required such that a machining amount in a corner portion is made equal to that in a linear portion. For example, Patent Document 1 proposes a technique of controlling a machining speed at the time of machining a corner portion according to an increase and a decrease of a machining removal distance (a machining allowance), for improving its machining accuracy. The machining removal distance is a length of a normal drawn from a point at an intersection of a discharge gap circle (a circle with a radius having a length obtained by adding a discharge gap to a radius of a sectional surface of the wire electrode) with the previous machining surface to the current machining surface, and is approximated to the machining amount described above. Further, Patent Document 2 proposes a technique of determining machining conditions by using a function, which uses a variable for a circular arc radius of a wire center trajectory for relatively moving a wire electrode, in order to suppress a shear drop (deviation) of a machining shape as well as a left over amount in a predetermined range.    Patent Document 1: Japanese Patent Application Laid-open No. 2004-148472    Patent Document 2: Japanese Patent Application Laid-open No. 2001-162446